A Problem with ‘Standard Candles’?

byPaul GilsteronMarch 19, 2010

Type Ia supernovae have become important ‘standard candles’ in judging cosmic distances, telling us how far away the host galaxy of a given supernova is. The idea here is that this kind of supernova produces a consistent luminosity because the white dwarfs that explode in the process are of uniform mass. The Type Ia supernova happens like this: A white dwarf gathers material from a companion star, growing in pressure and density so that the dwarf approaches the Chandrasekhar limit, beyond which it cannot support its own weight.

The result is a violent explosion that, like Cepheid variable stars, offers astronomers a way to gauge distances, and thereby to probe the shape of the cosmos at various distances and eras. Just how fast is the universe expanding, and in what ways? It was in 1998, prompted by supernovae of this kind, that the High-z Supernova Search Team discovered that the universe was not only expanding, but that its expansion was accelerating. Suddenly we were talking about ‘dark energy,’ which evidently operated against the gravitational force and seemed to offer a cosmic future in which the universe expanded without end.

Image: Cosmologists use Type Ia supernovae, like the one visible in the lower left corner of this galaxy, to explore the past and future expansion of the universe and the nature of dark energy. Credit: High-Z Supernova Search Team, HST, NASA.

It’s significant, given the importance of Type Ia supernovae in this analysis, that we’re now talking about exceptions to the Chandrasekhar limit, which is a critical mass of about 1.4 times that of the Sun. Four supernovae since 2003 have been discovered whose white dwarfs evidently surpassed the limit, the events being dubbed ‘super-Chandrasekhar’ supernovae. Richard Scalzo (Yale University) and a team of American and French physicists have measured the mass of one of the white dwarfs involved in the supernova SN 2007if, finding that it did indeed exceed the Chandrasekhar limit. Now work intensifies on modeling its structure.

Measuring the mass of the central star along with a shell of material ejected in the explosion and a surrounding envelope of pre-existing material, Scalzo and crew determined that the star had a mass of 2.1 times that of the Sun, significantly above the limit. So what’s going on here? In the paper on this work, Scalzo speculates that SN 2007if is the result of the merger of two white dwarfs, but other possibilities are also discussed in a paragraph that seems quite significant for our studies of dark energy:

The single-degenerate scenario… ensures that the white dwarf slowly approaches MCh [the Chandrasekhar mass limit] via accretion from a non-degenerate companion. In contrast, the double-degenerate scenario…, in which two white dwarfs in a binary system merge and explode, provides a way for SN Ia progenitors to exceed MCh and to give rise to more luminous events… There may therefore be a population of SNe Ia with a distribution of masses greater than MCh, with different explosion physics that interferes with luminosity standardization [italics mine].

And here is the nub of the problem:

The relative rate of such events among SNe Ia in general may also depend on redshift, and unless they can be identified or their luminosities accurately calibrated, they need not be common to produce significant biases in reconstructions of the dark energy equation of state.

Let me quote Scalzo as well from this Yale University news release on the same point:

“Supernovae are being used to make statements about the fate of the universe and our theory of gravity. If our understanding of supernovae changes, it could significantly impact our theories and predictions.”

And that’s why a paper studying a rare type of star that seems to ignore the Chandrasekhar limit swims into focus as we discuss a much broader issue. We’ve been speculating about the nature of dark energy for some time in these pages, even wondering if it might offer a clue to a far-future propulsion system by harnessing a force that appeared to operate against gravity. Now we find evidence that at least in some rare instances, the standard candles used in this work may be suspect. We need to find out how such supernovae can exist above the Chandrasekhar limit and learn whether they exist in sufficient numbers to compromise our measurements of the universe’s expansion.

The paper is Scalzo et al., “Nearby Supernova Factory Observations of SN 2007if: First Total Mass Measurement of a Super-Chandrasekhar-Mass Progenitor,” to be published in the Astrophysical Journal (preprint available).

Indictments of the ‘standard candle’ seem to be on the rise lately, but I believe that these sorts of challenges will turn out to be red herrings. We’ll perhaps know for sure once the LSST (Large Synoptic Survey Telescope) is operational and the number of recorded SN1a events out to z=1 will increase dramatically.

But let’s consider the argument that the Dark Energy hypothesis falls apart over the issue of the standard candle. A new problem ensues surrounding the issue of Dark Matter, which cosmologists and most astronomers still insist vastly outweighs the quantity of “normal” matter in the universe. Without Dark Energy, the universe would have been able to expand to its present size. Dark Energy is “needed” to overcome the cosmic gravitation of the purportedly vast quantities of Dark Matter — otherwise the universe would have halted its expansion eons ago and would have already collapsed in a “Big Crunch.”

So the bottom line is that if one wishes to dispose of the idea of Dark Energy, one also needs to dispose with Dark Matter. We can’t have one without the other or vice a versa. Challenging the standard cosmic model (the ‘concordance paradigm’) is all-or-nothing proposition. Indicting standard candles, however, only brings us half-way.

A complete overthrow of the standard model will require a reevaluation of a different observable — one that both Dark Energy and Dark Mater are predicated upon — namely: spectrographic radial velocities.

The authors of this paper, though, aren’t arguing for overthrowing the dark energy model. They’re saying that if a small segment of our standard candles needs reassessment, this could affect how we calibrate dark energy’s effects, and thus how we construct a theory that encompasses it. I’d see this more as tweaking the model rather than trying to eliminate it altogether.

“I’d see this more as tweaking the model rather than trying to eliminate it altogether.”

Exactly. Measuring cosmological distances is a tough problem so it matters whether we’re observing candles, matches or butane lighters, since we have so little else to go by. When something seems awry in our understanding, it is usually more profitable to study the light source more closely rather than to declare physics and cosmology to be all wrong.

Another reason why reassessing the type Ia supernovae for their standard candle qualification will at best slighly modify the dark energy picture is, that anything else would collide with the results of the measurements on the Cosmic Microwave Background (CMB) at ramming speed. The multi-year data results of the WMAP satellite (Wilkinson Microwave Anisotropy Probe, the latest release is the 7-year data) basically force you onto a spatially flat universe made up by roughly 70% dark energy and 30% matter, of which 95% are dark matter and the rest baryonic.

So, if my assessment of this work is correct, then the conclusion to be drawn is roughly this: the findings do not overturn or even challenge the notion that the Universe is accelerating, but they may limit our ability to distinguish between various competing explanations for the accelerating Universe.

I know there are other methods available for studying dark energy, such as baryon acoustic oscillations, the integrated saches wolfe effect, galaxy clustering, weak lensing, etc. that are independent of the supernova data. So, perhaps some of these other methods could “help pick up the slack” if the supernova data do not live up to their initial promise of high precision standard candles.

snactolate, we know what antimatter is and can produce it in our particle accelerators, though only in vanishingly tiny amounts. Dark matter is conjectural, and is thought to account for the anomalous rotation of spiral galaxies, among other observed phenomena (gravitational lensing around galaxy clusters is also in the picture, and explicable through dark matter). The point is, we don’t yet have an identification for what dark matter might be, though various theories exist, and searches are ongoing.

Thanks very much for the explanation… I did realize that particle accelerators like fermi were producing anti-matter (if only as a bi-product). So dark matter, dark energy and dark flow are all theoretical forces and states of energy(dark matter implies the existance of dark energy) and apparently must exist to maintain current physics? Does this imply the existance of a Dark Photon, Dark Fire, the whole opposite spectrum of light, energy and matter?

So, if my assessment of this work is correct, then the conclusion to be drawn is roughly this: the findings do not overturn or even challenge the notion that the Universe is accelerating, but they may limit our ability to distinguish between various competing explanations for the accelerating Universe.

I think it is even better than that, since there ought to be the possibility of identifying the relatively small number of “bad” candles, aliminate them from analysis, and thus gain in truth the accuracy that we previously only thought we had.

Not long after the announcement of the universe having an accelerating expansion, the great physicist John W. Wheeler (1911 -2008) argued that caution was in order. These kinds of measurements (given the sample sizes and distances involved) are quite difficult to make and the history of astronomy, strewn as it is with results that have to be revisited and corrected within a generation, should serve as a caution to the reasonable. Personally, I rather hope this new study has significant implications. By which I mean, I confess to having grown weary over the years of the Dark-this and Dark-that schools of cosmology when what in fact in needed is more work on the measurements (a lot more — and the theories underlying them) and a far less speculation. I hope that I am not alone in this. I would certainly not shed a tear over the demise of “Dark-energy” though I would be the last to make a prediction.

I did realize that particle accelerators like fermi were producing anti-matter (if only as a bi-product). So dark matter, dark energy and dark flow are all theoretical forces and states of energy(dark matter implies the existance of dark energy) and apparently must exist to maintain current physics? Does this imply the existance of a Dark Photon, Dark Fire, the whole opposite spectrum of light, energy and matter?

I wouldn’t bunch these items together. Dark matter can be inferred from observational data — galactic cluster microlensing is an example. We don’t know what it is yet, but we’re looking. Dark energy is needed to explain the continuing acceleration of the universe’s expansion. The ‘dark flow’ refers to the movement of particular galaxies in evident response to gravitational forces we haven’t yet identified. So we have three separate areas that are under intense investigation, but not three aspects of the same phenomenon.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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